Well, Obsidian would be OP if it were used in tools. But why not use it for other things? Such as armor, maybe? But obsidian armor would be a little too OP, yes. So why not make its health not-so-OP? Or you could just remove obsidian and put a mineral that isn't as good as obsidian, and is better than all the other minerals. If we have obsidian, why not use it for something? An obsidian knife is the least we could get..